1. Field of the Invention
The present invention relates generally to cardiac assist technologies which assist in maintaining a patient's cardiac output when the normal cardiac output is not sufficient to maintain an adequate pressure for supplying the patient's organs with arterial blood.
2. Description of the Prior Art
Recent statistics indicate that approximately ½ million Americans die of acute heart failure annually. Of these deaths, approximately 50% occur in spite of medical treatment (the other 50% do not reach the hospital). Although acute heart failure is presently treated with drugs and other therapy, present interventions are not sufficiently effective. As a result, additional measures are needed to help save lives of patients suffering from acute heart failure due to obstruction of the coronary vasculature or due to extensive cardiac surgery or other causes. Certain of these measures deal with varying or modulating coronary perfusion pressure according to the phases of the patient's cardiac cycle.
The coronary circulation system delivers blood to the heart muscle during the relaxation phase of cardiac contraction. During the contraction phase, pressure in the heart muscle rises and restricts coronary inflow, even though the arterial pressure rises due to cardiac ejection of blood. This elevation of coronary pressure increases the stiffness of the heart wall. With increased stiffness, the heart must expend more energy to bend the heart wall in order to eject blood. In other words, contraction of the heart against a large coronary pressure results in more “internal work” relative to the beneficial “external work” of ejecting blood from the heart chamber.
Theoretically, when the systolic coronary pressure is decreased, the heart wall becomes less stiff and can be more readily deformed during cardiac contraction. Thus, with reduced coronary pressure during the cardiac phase of contraction, the heart muscle requires less oxygen to overcome this important component of “internal” cardiac work. It is therefore often times desirable to vary or modulate coronary perfusion pressure according to the phases of a patient's cardiac cycle. Since perfusion pressure alters myocardial stiffness, changes in systolic stiffness should affect myocardial oxygen demand by changing the ratio of internal to external work. With a decreased systolic perfusion pressure, myocardial oxygen demands will be reduced, thereby permitting an increase in myocardial oxygen utilization efficiency.
A number of devices exist at the present time for varying or modulating coronary perfusion pressure. For example, the intra-aortic balloon catheter (IABC) is a commonly utilized ventricular assist device. This device is used when the patient's cardiac output is not sufficient to maintain an adequate arterial blood supply to the patient's organs. The IABC consists of an inflatable balloon attached to a catheter, which is advanced through the patient's femoral artery and into the descending aorta. Inflation and deflation of the balloon is accomplished by an external control unit synchronized with the heart beat. This unit rapidly inflates the balloon during the diastolic or resting phase of the heart cycle, and thus elevates diastolic aortic blood pressure and improves blood flow to the heart, the brain and other tissues. The balloon is rapidly deflated as the heart contracts. This action reduces the aortic blood pressure that the heart must overcome to eject blood from the left ventricle. Thus, the IABC is a ventricular assist device that also augments diastolic aortic blood pressure.
However, present IABC devices cannot sustain the circulation if the heart is severely diseased or injured, since ventricular ejection must be sufficient to keep the mean aortic blood pressure above approximately 60 mmHg. When the aortic pressure falls below this value, there is insufficient blood to fill the space around the balloon when it is deflated. In that case the wall of the aorta collapses around the deflated balloon of prior art devices, and the IABC becomes ineffective. Thus, present IABC devices can be used only in less severe cases of left ventricular failure.
Other technologies do exist that provide support for the ailing ventricles of the heart and include temporary or short term implant or pumping devices referred to in the industry as Left Ventricular Assist Devices (LVAD). These devices are designed to overcome the inherent design or physical limitations of IABC's but suffer the disadvantage of requiring very invasive surgical procedures, such as the thoracotomy, which consequently provides access to large chambers or conduits of blood within the body. The LVAD type devices also require significant surgical incisions and significant mechanical work in vital areas such as the aorta or heart chambers that may be a cause for complications or secondary events such as diminished mental acuity and cerebral injury now linked to the release of emboli into the brain.
A need therefore exists for a ventricular assist device which overcomes the various shortcomings of the prior art IABC and LVAD type devices presently in use.
A need also exists for a minimally invasive ventricular assist device which is capable of having multiple configurations within a patients body such that either left side support and/or right side support of the heart can be achieved using minimally invasive techniques. The device should be capable of providing true cardiac volumetric support and not merely act to unload the heart as in the case of IAB's. The device should function to actually displace clinically relevant blood volume like an LVAD but without the need for highly invasive surgical techniques. The result would be to provide a minimally invasive ventricular assist device.
Since clinical use of one device may be done while another device is also being used, a need exists for such an assist device which can also be used in conjunction with an intra-aortic balloon catheter IABC in a variety of clinical settings.